1. Field of the Invention
This invention relates to computing systems and more particularly to the real-time clock and configuration parameters utilized by computers.
2. Description of the Related Art
In prior art systems, the real-time clock was a battery-powered circuit providing date and time information independently from power supplied to the central processing unit (CPU) and other system components. When the computer is turned off, the real-time clock continues to update the time and date. In prior art systems, the real-time clock stores the time and date in a circuit on the motherboard of the computer called the CMOS RAM. The CMOS RAM also stores information relating to system configuration. The CMOS RAM is supplied with battery power to maintain configuration data even when the computer is powered off. During the boot process, the Basic Input/Output System (BIOS) utilizes information from the CMOS RAM, reads the time and date from the CMOS RAM, and sets an internal system clock used by software based on the time/date information.
Prior art implementations include the real-time clock and the CMOS RAM in a single device. Referring to FIG. 1, a prior art computer system 100 includes a CPU 102, connected to an I/O interface 104 that connects to integrated circuit 106. Device 106 includes a real-time clock 108, and CMOS RAM 110. CMOS RAM 110 includes storage locations 112 and 114 for date, time, and configuration data storage. A typical CMOS RAM implementation includes at least 64 bytes and may operate for several years with a single battery.
The bytes of the CMOS RAM can be addressed individually. Typically, the first 14 bytes of the CMOS RAM are reserved for time and date information and the control and status registers for the real-time clock. The remainder of the CMOS RAM stores hardware-specific information including processor clock speed, size of the BIOS ROM, data bus size, etc. The typical CMOS RAM includes four status registers that are used for monitoring and programming the operating mode of the real-time clock and the CMOS RAM chip. The real-time clock may be accelerated, slowed down, initialized, or adjusted for daylight savings time. In addition, various interrupts can be enabled. The CMOS RAM also stores information regarding the shutdown status of the computer and the indicators for memory and power failures. CMOS RAM 110 has typically been accessed in two ways: limited access via a BIOS interrupt 1Ah, and complete access via the address and data register accessible via input/output instructions at ports 70h and 71h. 
BIOS interrupt 1Ah provides access to date and time features of the real-time clock. Although CMOS RAM 110 includes additional data storage locations that are not part of the function of these date and time features, the additional data storage locations are not accessible via the BIOS interrupt handler for interrupt 1Ah. Instead, data stored in these additional data storage locations, e.g., configuration data, are accessed via the ports 70h and 71h. Port 70h accesses the address register of the CMOS RAM 110 and port 71h accesses the data register of the CMOS RAM 110. CPU 102 may access CMOS RAM 110 via the I/O interface 104, by accessing I/O ports numbered 70h and 71h. CPU 102 supplies an address to port 70h and retrieves the corresponding byte of data from port 71h. 
Device 106 is powered by battery 116. Although device 106 requires little power as compared to other memory circuits, it may need replacement after several years. Depending on the implementation, battery replacement may require merely opening the device package and replacing the battery, replacing the entire chip, or even replacing the motherboard. Replacement may be expensive and data may be lost in the process. Therefore, it would be desirable to replace device 106 with an implementation that does not require a battery power supply.